Giardia lamblia undergoes surface antigenic variation where only one of a set of variable specific surface antigens (VSPs) is expressed on the surface of each trophozoite at any one time. For reasons that are not known, one VSP is periodically exchanged for another. Although it is commonly taught that antigenic variation exists solely as an immune escape mechanism, antigenic variation also occurs in the absence of immune selection. A number of types of evidence suggests that biological selection also occurs. Prior experiments indicated that VSPs possess unique or different physical properties that under the appropriate conditions allow specific VSPs to be either positively or negatively selected. Most Giardia lamblia isolates can be adapted to grow in vitro in a complex media. However, Giardia can only be maintained under conditions favorable for mammalian cell culture for only a limited time period, only a few hours. Since Giardia adhere to the intestinal epithelium in vivo, it is important to understand their interactions. Results of studies to date have been limited because of inadequate culture conditions for either Giardia or mammalian cells or both. A system was developed that allowed Giardia to grow and multiply in the presence of epithelial cells for over a year. The WB isolate of Giardia was adapted to grow in a number of epithelial cells lines but multiple attempts to grow the GS isolate in the presence of epithelial cells were unsuccessful. This result lends support to the idea that the GS isolate of Giardia is so different from the WB isolate that it fulfills the criteria for being a new species. Giardia were continuously maintained for over a year through subculture and/or replacement of medium. The system required the presence of viable adhered epithelial cells and Giardia growth required a healthy monolayer. Different epithelial cell types supported growth to variable degrees. The type size, geometry and chemical make up of the culture vessel were important variables. Adapted Giardia grew faster than non-adapted Giardia. Certain VSPs were favored under in vitro conditions, a finding that supports biological selection. This system will allow detailed study of the interaction of Giardia and epithelial cells and why one VSP is favored over others under specific conditions. This laboratory has a long-standing interest in characterizing antigenic variation in Giardia lamblia including an understanding of the process, the biological advantage to the parasite and the consequences to the host. The surface of the trophozoite is covered by one of a family of related proteins called varying specific surface protein (VSPs) that change. Even though these proteins have certain common motifs such as an absolutely conserved cytoplasmic tail, CRGKA, their external residing portion is antigenically distinct and they have individual biochemical differences. As a consesquence both biological as well as immunological selection has been documented in vivo and in vitro. This laboratory has a long-standing interest in characterizing antigenic variation in Giardia lamblia including an understanding of the process, the biological advantage to the parasite and the consequences to the host. The surface of the trophozoite is covered by one of a family of related proteins called varying specific surface protein (VSPs) that change. Even though these proteins have certain common motifs such as an absolutely conserved cytoplasmic tail, CRGKA, their external residing portion is antigenically distinct and they have individual biochemical differences. As a consesquence both biological as well as immunological selection has been documented in vivo and in vitro. The repertoire of VSP in the isolate WB was determined from previously published and newly obtained vsp sequence and their structure, relatedness and organization in the genome determined. There were 228 complete and 75 partial vsp gene sequences for an estimated repertoire of 270 to 303, making up about 4% of the genome. The vsp gene diversity included 30 genes containing tandem repeats, and 14 vsp pairs of identical genes present in either head-to-head or tail-to-tail configurations (designated as inverted pairs), where the two genes were separated by 2 to 4 kb of non-coding DNA. Interestingly, over half the total vsp repertoire was present in linear gene arrays that can contain up to 10 vsp gene members. Not all the VSP possessed classic Giardia Zn fingers or GGCY motifs. Three major groups of VSPs were identified. Although there are similar features among VSPs, VSPs also differ greatly, which may explain the antigenic diversity of VSPs and the physical chemical differences. The latter likely leads to biological selection in vivo. The mechanisms of catalysis and inhibition of the Giardia lamblia arginine deiminase was explored. The protein was essential for survival. Because Giardia lamblia possess the arginine dihydrolase pathway to generate ATP and humans do not, the enzymes of this pathway are a potential drug target for the treatment of human giardiasis The pathway includes the enzymes arginine deiminase, ornithine transcarbamoylase and carbamate kinase (CK). The crystal structure of enzyme CK was determined and the enzyme characterized